Quasi crystals
Quasi crystals

Research at Brown: Gregory Crawford: Switchable quasi-crystal structures with five, seven, and nine-fold symmetry

Switchable quasi-crystal structures with five, seven, and nine-fold symmetry
Quasi crystals with 5, 7 and 9 fold symmetry in the holographic polymer dispersed liquid crystal (H-PDLC) have been demonstrated. Quasi-crystal structure in H-PDLC were investigated in an attempt to understand, explore and design new optical properties for advanced electro-optical devices.

Classical crystallography strictly defines crystal structure as repeating pattern that has both translation and rotational symmetry which limits the symmetry to only 2, 3, 4 and 6-folds. Other symmetries were assumed to be impossible to form crystalline structures until quasi-crystals were discovered in several transition metal alloys. Quasi-crystals exhibit multi-fold symmetries of any number but lack an overall absolute translational symmetry. Every since the discovery of quasi crystals the scientific community had been searching for extraordinary properties in these unusual crystals. The H-PDLC mixture between the conducting glass substrates is exposed to interference pattern generated by N-beams with the same polar angle and equally distributed along an azimuth plane to produce quasi-crystal in H-PDLC. During the exposure photo polymerization occurs in the bright regions forcing the LC to diffuse into the dark regions. The LC is spatially positioned in the low intensity region bounded by the polymer matrix in the high intensity regions permanently recording the quasi-crystal pattern. The refractive index mismatch between polymer and enclosed LC results in the formation of a diffraction grating with causes strong diffraction of light passing through it. The sample, when probed with a HeNe laser, produces rings of spots at different cone angles around the zero order central spot. The sample diffracts white light passing through it producing a colorful spectrum in the radial direction. The scanning electron microscope image and diffraction pattern in conjunction with diffraction theory and Fourier techniques were used to prove the existence of quasi periodicity in these samples. The quasi crystal grating in H-PDLC can be switched by applying electric field across the film. As in the metal alloys, H-PDLC quasi-crystals are expected to have interesting thermo and mechanical properties. These lattice structures have already shown relatively polarization-insensitive optical characteristics compared to conventional lattice structure H-PDLCs.

Brown faculty collaborators:


Other project collaborators:

Suraj Gorkhali
Division of Engineering, Brown University, Providence, RI Back to Gregory Crawford's Brown Research Profile
Research at Brown: Gregory Crawford: Switchable quasi-crystal structures with five, seven, and nine-fold symmetry
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